This invention relates to multilamp photoflash devices having circuit means for igniting the flashlamps and, more particularly, to high-voltage photoflash arrays with improved means for providing electrostatic protection.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art; particularly, in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226; 3,912,442; 3,935,442; 3,937,946; 3,941,992 and 3,952,320 and referred to as a flip flash) employs high-voltage type lamps adapted to be ignited sequentially by successively applied high-voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element. The flip flash unit comprises a planar array of eight high-voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves respectively of the rectangular-shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of four lamps. The application of successive high-voltage pulses (e.g., 500 to 4,000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the four lamps at the upper half of the array to be sequentially ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps.
The flip flash circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including the terminal contacts, on one side. The flashlamp leads are electrically connected to these circuit traces by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally open, connect switches that chemically change from a high to low resistance, so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of these switches is to provide lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in parallel with three of the four lamps being connected in series with their respective thermal connect switches. Initially, only the first of the group of four lamps is connected directly to the voltage pulse source. When this first group flashes, it causes its associated thermal connect switch (which is series connected with the next, or second, lamp) to become permanently conductive. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed.
The primers used in the high-voltage type flashlamps employed in such arrays are designed to be highly sensitive toward high-voltage breakdown. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamps. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern miniature cameras. The mechanical energy delivered to the piezoelectric crystal, and thereby the electrical output energy therefrom, is limited, both by the size of the device and by the necessity to minimize camera vibration and motion during use.
The high degree of electrical sensitivity needed in high-voltage flashlamps gives rise to distinct problems of inadvertent flashing during handling of the array package. Any static charges on equipment and personnel can cause the lamps to flash. This problem is discussed in the aforementioned U.S. Pat. No. 3,935,442, and one means described therein for protecting against inadvertent flashing is to make the reflector member electrically conductive, such as fabricating it of metal or metal-coated plastic and electrically connecting the reflector to an electrical "ground" portion of the circuitry on the circuit board. Thus, the reflector member functions as an electrical shield and increases the stray capacitance to ground of the electrical "ground" of the circuitry, reducing the possibility of the accidental flashing of the lamps by electrostatic voltage charges on a person or object touching the array.
A further approach described in the aforementioned U.S. Pat. No. 3,941,992 for providing electrostatic protection is to metalize the back surface of the circuit board and connect that metalized surface to the common circuit conductor run, for example, by means of an eyelet through the board, thereby providing a planar conductive shield behind the lamps and circuitry.
An improved method for providing electrostatic protection at the back of the array is described in copending application Ser. No. 644,674, filed Dec. 29, 1975 and assigned to the present assignee. A conductive shield of planar configuration is spaced from the back surface of the circuit board and connected to the common circuit conductor. The use of such a common-connected conductive shield between the hot lamp leads (and their attachment means) and the back surface of the array package has been observed to significantly reduce the probability of inadvertent lamp flashing due to externally applied electrostatic charges at the rear surface of the array. In a preferred embodiment, a conductive coating is applied to a flash indicator insert on the back of the array. Electrical contact with the common circuit conductor is achieved by way of the common lead-in wires of the lamps and eyelets that extend through the rear of the circuit board. The extension, or projection, of these leads and eyelets at an angle from the rear of the board serves to provide contact with the conductive corona shield layer and maintain a spacing between the shield and the circuit board itself. In order to prevent short circuiting of the lamps, the "hot" eyelet (and lead-in wire) of each lamp is bent up tightly in contact with the rear surface of the circuit board.
Although the above-described protective packaging features are effective with respect to various sources of electrostatic discharges, we have observed that commercial arrays employing such shield elements are still subject to inadvertent electrostatic-caused flashing of the lamps contained therein. More specifically, testing has shown that these prior art approaches do not eliminate all of the charges that can build upon the front face of the array and sometimes discharge through the lamps and result in an inadvertent flash.
While the mere fact of electrically connecting the reflectors to the common circuit conductor (e.g., by means of a spring clip) reduces the number of electrostatic-induced inadvertent lamp flashes, the protection so offered is far from complete. Contact of a charged object, such as the user's hand, with the front face of the array causes a high induced charge on the combustible shreds of the lamps contained therein. This can result in electrical discharge from lead-in wires through the primer coating to the shreds inside the lamp and subsequent lamp ignition. Tests have shown as high as 30% lamp ignitions when a 10,000 volt field is placed between the front face of the array and the exposed plug-in tab at the end of the array.